It is well known in the art that following a start-up from cold, that is to say, an engine start-up where the temperature of the engine is close to ambient temperature, significant losses are produced due to the lubricating oil being below an optimum operating temperature. These losses increase fuel usage during the initial warm-up period and in addition wear is increased if the oil is below a minimum temperature at which additives in the oil become fully activated.
Several methods have therefore been previously proposed to either actively heat the oil by the use of electric oil heaters or by heat transfer with the exhaust gas from the engine or by passive heating by recirculating at least some of the oil that has already passed through the engine thereby speeding up the heating of the oil by the use of partitioned oil reservoirs such as that shown in published patent application GB-A-2251889.
It is a problem with all such previous attempts that, although the temperature of the oil may be increased by these measures before it enters the engine, the very large thermal mass of the engine compared to the thermal mass of the oil means results in the temperature of the oil rapidly reducing as it flows through the engine by the transfer of heat from the oil to the engine. Therefore, by the time the oil reaches the key components of the engine requiring lubrication such as the main bearing of the crankshaft its temperature will normally be close to the temperature of the engine components through which it has passed. In addition, the high thermal mass of the engine means that it will take several minutes for the engine to attain its normal operating temperature of approximately 90° C. after a cold start and during this period of time the oil will likely be more viscous and may have lower lubricating properties than are desirable for optimum fuel efficiency. Although this is a particular problem following a start-up from cold it can be a persistent problem with some engines if the oil passages to the bearings are located in a cool part of the engine where the temperature of the engine during normal running of the engine remains below that required for optimum oil operation.